1. Field of the Invention
This invention relates to a radio communication system and a frame synchronizing method performed between base stations, designed to achieve excellent radio frame synchronization.
2. Description of the Related Art
FIG. 5 is a schematic diagram showing a conventional radio communication system. In the drawing, a reference numeral 1 denotes a an ISDN network; 2 a switchboard; 3 a GPS (Global Positioning System) receiver for receiving a GPS reference signal (PPS signal) from a GPS satellite; 4 an absolute base station which adjusts the phase of a frame signal based on the PPS signal received by the GPS receiver 3 when a predetermined re-synchronization time is reached, and transmits the phase-adjusted frame signal to subordinate base stations 5 and 6; and 5 and 6 the subordinate base stations, each of which adjusts, upon reception of the frame signal from the absolute base station 4, the phase of an internal frame signal therein to coincide with the phase of the received frame signal.
FIG. 6 is a flowchart showing a conventional frame synchronizing method performed between base stations.
Next, an operation will be described.
The phase of a frame signal in the absolute base station 4, and the subordinate base stations 5, and 6 depends on a line clock transmitted from the ISDN network 1 through the switchboard 2. In other words, the phase of a frame signal is decided based on an internal clock operating according to the line clock.
Therefore, the synchronization of a frame signal is maintained as long as the absolute base station 4 and the subordinate base stations 5 and 6 use the same line clock.
However, if there is caused a discrepancy between the line clock and the PPS signal, an error is generated in the internal clocks at each of the absolute base station 4 and the subordinate base stations 5 and 6, thereby making it difficult to maintain the synchronization of the frame signal. Therefore, it is necessary to correct the internal clocks at regular intervals.
Thus, when a predetermined re-synchronization time is reached (step ST1), if a call is not in a connected state (step ST2), the absolute base station 4 corrects the error of the internal clock based on a PPS signal received each time by the GPS receiver 3, and adjusts the phase of a frame signal. In other words, the phase of a frame signal is adjusted such that the head of the frame signal can coincide with the rising edge of the PPS signal.
Upon reception of the phase-adjusted frame signal from the absolute base station 4, each of the subordinate base stations 5 and 6 adjust the phase of the internal frame signals thereof to coincide with the phase of the phase adjusted frame signal (step ST3).
If the call is in a connected state, processing is set to the standby mode until the call is disconnected, and once the call is disconnected, the phase of the frame signal is adjusted.
Because of the foregoing configuration of the conventional radio communication system, if the accuracy of the line clock is high, and the discrepancy between the line clock and the PPS signal is small, then the synchronization of a frame signal can be maintained even without correcting the error of the internal clock dependent on the line clock. However, if the accuracy of the line clock is low, and the error of the internal clock is made large, then regarding the management of re-synchronization time of the absolute base station, no coincidence is reached among absolute base stations, making it impossible to perform simultaneous re-synchronization at the same time. Therefore, there has been such an inherent problem in the conventional art that the maintenance of the synchronization of a frame signal is not made possible.
Another problem inherent has been the impossibility of synchronizing the frame signal until the call is disconnected, if the call is in a connected state at the time of re-synchronization.